1. Field of the Invention
The present invention relates to a refrigeration cycle and refrigerant compressor, and it relates to, in particular, a refrigeration-cycle-constituting material system comprising a refrigerating machine oil composition suitable for a flon type refrigerant containing no chlorine and having a critical temperature of 40.degree. C. or higher, for example, flon 134a, and electrical insulating materials and a drying agent which are hardly deteriorated by the refrigerating machine oil composition.
2. Prior Art
In recent years, chlorine-containing flons (chlorofluorocarbons, abbreviated as CFC) have been included in the list of compounds under regulation in use, all over the world because of the problems of environmental pollution, in particular, the ozone depletion and the global warming.
All of flons included in the list of compounds under regulation in use are chlorine-containing flons such as flon 11, flon 12, flon 113, flon 114, flon 115, etc. Flon 12 which has been exclusively used as a refrigerant in refrigerating apparatus such as refrigerators, dehumidifiers, etc., has also been included in the list.
Therefore, a refrigerant usable in place of flon 12 is required. Hydrofluorocarbon (HFC) having a low reactivity with ozone and a short decomposition period in the air has recently been noted as a substitute refrigerant. Flon 134a (1,1,1-tetrafluoroethane, CH.sub.2 FCF.sub.3) is a typical example of such a refrigerant. In detail, when the ozone depletion potential (ODP) of flon 12 (dichlorodifluoromethane CCl.sub.2 F.sub.2) is taken as 1, that of flon 134a is zero. When the global warming potential (GWP) of flon 12 is taken as 1, that of flon 134a is 0.3 or less. Flon 134a is noncombustible and similar to flon 12 in thermal properties such as temperature-pressure characteristics. Therefore, flon 134a has been said to be advantageous in that it can be put into practical use without greatly changing the structures of refrigerating apparatus such as refrigerators and dehumidifiers and refrigerant compressors in which flon 12 has heretofore been used.
Flon 134a, however, has a unique chemical structure and hence very characteristic properties. Therefore, it has a very poor compatibility with refrigerating machine oils such as mineral oils and alkylbenzene oils which have been used in conventional refrigeration system using flon 12, and hence it cannot be put into practical use at all. In addition, the suitability including the improving effect on the lubrication and the resistance to frictional wear of the sliding portions of compression mechanical parts, the influence on electrical insulating materials, the influence on drying agents, etc. is a problem, and there has been an eager desire for the development of a novel material system constituting a compressor and a refrigerating apparatus.
Therefore, before referring to the problem of the miscibility of a refrigerant with a refrigerating machine oil, conventional refrigerant compressor and refrigeration apparatus which use a flon type refrigerant are first explained with reference to FIG. 7 to FIG. 9.
FIG. 7 is a vertical cross-sectional view of the principal part of a conventional closed rotary compressor. FIG. 8 is a cross-sectional view for explaining the displacement volume of the compressor section of the compressor. FIG. 9 is a diagram showing the structure of an ordinary refrigeration cycle.
In FIG. 7, numeral 1 shows a case used both as a closed container and as a oil pan. In the case 1, an electric motor section 22 and a compressor section 23 are accommodated.
The electric motor 22 is composed of a stator 19 and a rotor 20, and a rotating shaft 4A made of cast iron is fitted in the rotor 20. The rotating shaft 4A has an eccentric portion 3 and an shaft hole 17 is formed in hollow form on the one side of the eccentric portion 3.
The core wire of the winding wire portion 19a of the stator 19 is coated with an ester imide film, and an electrical insulating film of a polyethylene terephthalate is inserted between the core portion and the winding wire portion of the stator. The rotor 4A has a surface finished by grinding.
The compressor 23 has as its chief mechanism components a cylinder 2 made of an iron-based sintered product; a roller 7 made of cast iron which is fitted in the eccentric portion 3 of the rotating shaft 4A and eccentrically rotated along the inside of the cylinder 2; a high-speed steel vane which is reciprocated in the groove 8 of the cylinder 2 while one side of the vane is in contact with the roller 7 and the other side is pushed by a spring 9; and a main bearing 5 and a sub-bearing 6 which are made of cast iron or an iron-based sintered product, are provided on both ends of the cylinder, and serve both as bearings for the rotary shaft 4A and as the side wall of the cylinder 2.
The sub-bearing 6 has a discharge valve 27, and a discharge cover 25 is attached thereto so as to form a silencer 28. The main bearing 5, the cylinder 2 and the sub-bearing 6 are fastened with a bolt 21.
A pump chamber 12 is composed of a space and parts surrounding the space, i.e., the back of the vane 10, the groove 8 of the cylinder 2, the main bearing 5 and the sub-bearing 6.
The main bearing 5 has a suction piece 14 which can suck a naphthene type or alkylbenzene type refrigerating machine oil 13A in which a refrigerant flon gas stored in the bottom of the case 1 has been dissolved, into the pump chamber 12. The sub-bearing 6 has a discharge port 16 which can discharge the refrigerating machine oil 13A to an oil tube 15 from the pump chamber 12. The oil tube 15 is designed to be able to supply the refrigerating machine oil 13A to the shaft hole 17 of the rotating shaft 4A and then to predetermined sliding portions from the shaft hole 17 through a branch opening 18.
The action of the rotary compressor thus composed is explained below with reference to FIGS. 7 and 8. When the compressor is operated to rotate the rotating shaft 4A made of cast iron, a roller 7 made of tempered cast iron is rotated with the rotation of rotating shaft 4A, and the high-speed steel vane 10 is reciprocated in the groove 8 of the cylinder 2 made of cast iron or a iron-based sintered product while the vane 10 is pushed by the spring 9 and its end is in contact with the roller 7. Thus, the vane 10 compresses a refrigerant (flon 12) which has flown in through a refrigerant suction opening (not shown), and the refrigerant is discharged outside the compressor from a discharge pipe 29 through a refrigerant discharge opening 24. The winding wire portion 19a and the electrical insulating film (not shown) of the stator 9 are immersed in the refrigerating machine oil containing flon dissolved therein, or they are exposed to circumstances of spraying with mist of the refrigerant oil.
In the case of a combination of a conventional refrigerating machine oil consisting of a mineral oil or an alkylbenzene and flon 12, flon 12 is completely miscible with the refrigerating machine oil in all use ranges, so that it has been not necessary at all to care about the various problems concerning the miscibility of flon 134a with a refrigerating machine oil which are hereinafter described in detail, namely, the separation into two layers between the refrigerating machine oil and the refrigerant in a compressor, and the residence of the refrigerating machine oil in a heat exchanger. However, in the case of fluorohydrocarbon type refrigerants containing no chlorine which have unique characteristics, for example, flon 134a, the miscibility of the refrigerant with a refrigerating machine oil is the most serious problem in practice because there is no practical refrigerating machine oil which can easily dissolve the refrigerant.
In general, for improving the performance characteristics of a compressor, namely, the coefficient of performance (COP) which indicates the energy efficiency, it has been necessary to minimize the mechanical loss of the compressor and maximize its volumetric efficiency.
The mechanical loss of a refrigerant compressor mainly includes the friction loss at the journal bearing and thrust bearing in the mechanical part and the power for agitating oil. In general, it has been said that the best means is to minimize the value of the coefficient of friction (.mu.) defined by the following equation on the basis of the hydrodynamic lubrication theory of a journal bearing: EQU .mu.=2.pi..sup.2 (D/C).eta.N/P (9)
wherein
N: revolution rate, PA0 P: pressure on surface, PA0 .eta.: viscosity, PA0 D: diameter of shaft, PA0 C: diametral clearance.
This fact indicates that in a refrigerant compressor operated under hydrodynamic lubrication conditions, not only the structural factors regarding dimensions and shapes but also the actual viscosity of a refrigerating machine oil containing flon disolved therein which is a factor influenced by operation circumstances, have a close relationship to the mechanical loss of the compressure.
On the other hand, for keeping the volumetric efficiency highest, it is necessary that in a mechanical chamber for compressing a refrigerant gas, the leakage of the refrigerant gas from the high pressure side to the low pressure side should be prevented by carrying out sealing between parts which works to compress the refrigerant gas. It should be noted that also in this case, the actual viscosity of a refrigerating machine oil containing the refrigerant dissolved therein has an important function.
As described above, in a refrigerant compressor heretofore used by the use of flon 12 and a refrigerating apparatus using the refrigerant compressor, it is important for the improvement of performance characteristics of the compressor to optimize the actual viscosity of a refrigerating machine oil containing the refrigerant dissolved therein, at a rated operation point under usual operation conditions.
A refrigerating apparatus such as a refrigerator or a dehumidifier is operated, though in rare cases, in a high-temperature circumstance much more severe than usual operation conditions. In this case, the lubrication in the apparatus gets into a so-called boundary lubrication region in which a lubricating oil layer is thined, so that the metal surfaces of sliding portions of a bearing are brought into contact with each other. Consequently, the coefficient of friction is increased at once, resulting in heat generation. Therefore, scoring or seizing-and-adhesion phenomenon occurs between the bearing and a rotating shaft and deteriorates the reliability of a refrigerant compressor. Therefore, some consideration is needed for preventing a fatal problem from occurring even under boundary lubrication conditions. In a conventional refrigerant compressor using flon 12, chlorine in flon 12 acts effectively as an extreme pressure agent. In detail, when scoring or seizing-and-adhesion phenomenon takes place between a bearing and a rotating shaft, the refrigerant flon 12 dissolved in a refrigerating machine oil as bearing-lubricating oil is decomposed by frictional heat generated by the scoring or the phenomenon, and chlorine, i.e., the decomposition product, reacts with iron on the surface of the bearing to form iron chloride which acts as a lubricant.
As described above, in the case of a refrigerating apparatus using a high-pressure vessel type rotary compressor, for example, a refrigerator, a refrigerant compressor and a refrigerating apparatus which satisfy the operation conditions at an ambient temperature of 30.degree. C. described below are satisfactory in the coefficient of performance indicating energy efficiency and the reliability of a product, and most products have been used in such ranges. The discharge pressure of the compressor: about 10 kg/cm.sup.2 abs, oil temperature: about 100.degree. C., refrigerating machine oil: an alkylbenzene oil or a mineral oil having a viscosity at 40.degree. C. of 56 cSt and a viscosity at 100.degree. C. of 6 cSt, the actual viscosity of which becomes 1 to 4 cst.
On the other hand, in the case of a refrigerating apparatus using a low-pressure vessel type reciprocating compressor (the explanation of the structure and operation is omitted), for example, a refrigerator, there have been used a refrigerant compressor and a refrigerating apparatus which satisfy the following operation conditions at an ambient temperature of 30.degree. C.; the suction pressure of the compressor: about 1.6 kg/cm.sup.2 abs, oil temperature: 85.degree. C., refrigerating machine oil: a mineral oil having a viscosity at 40.degree. C. of 8 to 15 cSt and a viscosity at 100.degree. C. of 1.8 to 4.2 cSt, the actual viscosity of which becomes 2 to 6 cSt.
Next, a fundamental refrigeration cycle provided with a refrigerant compressor which thus sucks, compresses and then discharge a flon type refrigerant, is explained below with reference to FIG. 9.
As shown in FIG. 9, a compressor 40 compresses a low-temperature, low-pressure refrigerant gas, discharges the resulting high-temperature, high-pressure refrigerant gas and send the same to a condenser 41. The refrigerant gas sent to the condenser 41 becomes a high-temperature, high-pressure refrigerant fluid while releasing its heat to the air, and then it is sent to an expansion mechanism (e.g. an expansion valve or a capillary tube) 42 while being freed from water by a dryer 45. The high-temperature, high-pressure refrigerant fluid which passes the expansion mechanism becomes low-temperature, low-pressure wet vapor owing to squeezing effect and is sent to an evaporator 43. The refrigerant introduced into the evaporator 43 is evaporated while absorbing heat from the surroundings, and the low-temperature, low-pressure gas which has come out of the evaporator 43 is sucked into the condenser 40. Thereafter, the above cycle is repeated.
As the frigerant, flon 12 has heretofore been used. However, the employment of flon 12 is under regulations, as described above. The employment of flon 134a in place of flon 12 involves many problems because conventional mineral oil type or alkylbenzene type refrigerating machine oils for flon 12 are very poor in miscibility with flon 134a. Therefore, refrigerating machine oils having a good miscibility with flon 134a have been vigorously developed and various refrigerating machine oils have been proposed. As typical examples of such refrigerating machine oils, there are known the compounds having ether linkages exemplified below.
For example, Japanese Patent Application Kokai No. 1-259093 discloses "a refrigerating machine oil for a flon compressor" which comprises as base oil a propylene glycol monoether represented by the general formula: ##STR2##
wherein R is an alkyl group having 1 to 8 carbon atoms, and n is an integer of 4 to 19; Japanese Patent Application Kokai No. 1-259094 discloses a diether type compound obtained by etherifying one end of propylene glycol which is represented by the general formula: ##STR3##
wherein each of R.sub.1 an R.sub.2 is an alkyl group having 1 to 8 carbon atoms, and n is an integer (average molecular weight: 300 to 600); and Japanese Patent Application Kokai No. 1-259095 discloses a monoether type compound which is a copolymer of propylene glycol and ethylene glycol and is represented by the general formula: ##STR4##
wherein R is an alkyl group having 1 to 14 carbon atoms, and m and n are integers, the ratio m:n being 6:4 to 1:9 (average molecular weight: 300 to 2,000).
The difference of these polyalkylene glycols from conventional mineral oils and alkylbenzene oils have been reported as follows. By the introduction of ether linkages into the molecule, the affinity for flon 134a is enhanced to improve the miscibility with flon 134a greatly, refrigerant lubrication due to the phenomenon of separation into two layers (a phenomenon that the refrigerant and the refrigerating machine oil are insoluble in each other and separate; hereinafter referred to merely as "two-layer separation") in the sliding portions of a compressor can be prevented, the return of the oil to the compressor which is induced by residence phenomenon due to the adhesion of the oil to the inner wall of a heat exchanger can be suppressed, and there can be solved the problems concerning the reliability of the compressor and a refrigerating apparatus, for example, seizing and scoring in the sliding portions of the compressor.
Such compounds thus containing a large number of ether linkages (C--O--C), however, are disadvantageous in that,
(1) they have a saturation water absorption rate is high (they tend to absorb water). PA1 (2) they have a low volume resistivity. PA1 (3) they have a low oxidation stability, so that the total acid value is apt to be increased.
Therefore, the compounds have been not suitable for refrigerant compressors and refrigerating apparatus in which a hermetic motor is used as an electric motor. That is, although the compounds have an improved miscibility with the refrigerant, they are disadvantageous in that they attack the insulating materials of the motor to deteriorate the electrical insulating characteristics. In all of the above compounds, the end group having an ether linkage is capped with hydrogen, and the hydrogen further increases the hygroscopicity. Therefore, it has been proposed to replace the hydrogen by an ester group to obtain a refrigerating machine oil represented by the following formula (see Japanese Patent Application Kokai No. 2-132178): ##STR5##
wherein R is a hydrocarbon group, R.sup.1 is an alkylene R.sup.2 is an alkyl group, and n is an integer which is such that the viscosity of this compound becomes 10 to 300 (at 40.degree. C.).
However, the improved miscibility with the refrigerant of this compound is also brought about by a large number of ether linkages in the molecule, like that of the above compounds, and hence this compound involves the same problems as in the case of the above compounds.
Thus, the compounds having ether linkages tend to absorb water because of the above problem (1), and the compounds themselves are hydrolyzed by the water to become unstable. Furthermore, the water freezes, chokes the capillary of a refrigeration cycle, and disturbs the pressure balance. The volume resistivity of the compounds is low as described as the problem (2), so that the electrical insulating properties are deteriorated. When the total acid value is increased as described as the problem (3), the compounds are hydrolyzed to become unstable.
As described above, flon 134a which is used as a substitute refrigerant for conventional refrigerant flon 12 involves the following fatal problem. Because of its unique molecular structure, flon 134a has a low affinity for mineral oil type and alkylbenzene oil type refrigerating machine oils which have heretofore been used, and hence it lacks miscibility with the refrigerating machine oils which is essential in a refrigerant compressor and a refrigerating apparatus.
Attempts have been made to improve the miscibility, but have been accompanied with, for example, the deterioration of the electrical insulating properties, the water problem, and the unstability problems, such as the hydrolysis and the decomposition of the compound by an acid. Each problem is described below in more detail.